Method of manufacturing a cam for use in an ignition distributor

ABSTRACT

A cam for use in a distributor in a road vehicle is made by grinding the cam to the desired profile in the usual way, this operation leaving the cam surface with sharp edged asperities defining therebetween channels which in use store lubricant. The cam is then placed in a container with abrasive particles and the container is so moved that the particles collide with the cam surface and round off the sharp edges of the asperities without removing the asperities.

United States Patent Inventor John Snell Welford-On-Avon, England Appl. No. 789,504 Filed Jan. 7, 1969 Patented May 25, 1971 Assignee Joseph Lucas (Industries) Limited Birmingham, England Priority Jan. 8, 1968 Great Britain 981/68 METHOD OF MANUFACTURING A CAM FOR USE IN AN IGNITION DISTRIBUTOR 1 Claim, 3 Drawing Figs.

US. Cl 51/315, 51/318, 51/326 Int. Cl B24b 1/00 Field ofSearch 51/315, 318, 326

, [56] References Cited UNITED STATES PATENTS 2,548,264 4/1951 Howe 51/315 2,932,205 4/1960 Keesling 51/315X 3,482,357 12/1969 Inaba et al. 51/105 Primary ExaminerLester M. Swingle Attorney-Holman and Stern ABSTRACT: A cam for use in a distributor in a road vehicle is made by grinding the cam to the desired profile in the usual way, this operation leaving the cam surface with sharp edged asperities defining therebetween channels which in use store lubricant. The cam is then placed in a container with abrasive particles and the container is 'so moved that the particles collide with the cam surface and round off the sharp edges of the asperities without removing the asperities.

PATENTEnuAvzslsn V 3579.933

' sum 1 OF 2 INVEN E ATTOEN EYS PATENTEU HAY25 I97! SHEET 2 [IF 2 REQQ Q JNVNTZTOE a? w ATTOEN EYS METHOD OF MANUFACTURING A CAM FOR USE IN AN IGNITION DISTRIBUTOR This invention relates to cams for ignition distributors of the kind including a shaft adapted to be driven by the engine of a vehicle with which the distributor is associated, the cam being rotatable with the shaft, and a cam follower which engages said cam to effect opening and closing of a pair of contacts.

When a distributor of the kind specified is in operation, the cam gradually wears the cam follower away, but in the past the wear rate has been extremely extremely small and of no significance.- However, it has recently been proposed to use synthetic resin cam followers which have the advantage that they are less expensive to manufacture than conventional than conventional cam followers, and furthermore are lighter in weight so that the risk of contact bound at high engine speed is minimized. Such cam followers have been extremely successful, but in some cases have been found to wear at an unacceptably high rate. The present invention seeks to overcome this problem.

It is of course conventional to grind a cam for use in a distributor to a reasonably fine finish, and at first sight it may appear that all that is required in order to avoid the high wear rate which is experienced with synthetic resin cam followers is to grind the cam to a finer finish than conventional cams. There are a number of reasons why this procedure is unsatisfactory. Thus, in order to grind a cam to a very fine finish, it is necessary to replace the grinding wheel with a finer wheel at least once, and moreover the overall grinding time is increased so much that the cost of the cam rises sharply. Secondly, it is vital that a cam should be able to store lubricant within its surface, otherwise the wear rate increases very rapidly indeed. If a ground cam is inspected under a microscope, it is found to consist of a large number of sharp edged asperities of varying heights between which are defined channels. These channels store the lubricant in use, and if the cam is ground excessively, the channels; gradually become smaller, and so that amount of lubricant that can be stored is considerably reduced. Quite apart from these two points, it has been found that the major contributory factor to the wear of the cam follower is the sharp edges of the asperities, which act as cutting edges in a manner somewhat analogous to the action of a coal cutter pick. Grinding of the cam removes some asperities, but leaves other asperities with their sharp edges which can still cause damage. Moreover, the grinding operation at the same time reduces the volume of the channels, so that less lubricant is stored, which worsens the problem.

Once it is appreciated that the damage to the cam follower is being caused by the sharp edges of the asperities, an alternative process for finishing the cam which suggests itself is a polishing operation, does not leave further sharp edges, and unlike a grinding operation, doe not leave further sharp edges in their place. Unfortunately, the polishing operation, like the grinding operation, has the disadvantage that it still reduces the total volume of the channels so that less lubricant is stored, and so this process is also unsatisfactory.

The present invention is based on the realization that in order to reduce the wear rate the sharp edges of the asperities must be removed without substantially reducing the total volume of the channels, and resides in a method of forming a cam, comprising grinding the cam to the desired profile, the grinding operation leaving the cam surface with sharp edged asperities defining therebetween channels which in use store lubricant, placing the cam in a container with abrasive particles, and moving the container so that the particles collide with the cam surface and round off the sharp edges of the asperities without removing the asperities.

It will be appreciated that the round off operation itself can be performed using a number of well-known machines. However, the invention resides not only in the solution to the particular problem, but the identification of the problem itself.

of the invention, and

FIGS. 2 and 3 respectively are micrographic representations of the surface of a cam, FIG. 2 showing the cam after it has been ground to the desired profile but before the finishing operation, and FIG. 3 showing the same cam after the finishing operation.

In forming a cam, a distributor cam shaft of the desired length is cut from bright drawn bar and is then machined in the usual manner to leave a shaft of the required dimensions having thereon a cam which is ground to the desired profile. A plurality of such shafts are then subjected to a finishing operation in a vibratory finishing machine. This machine is well known in itself, and is not therefore shown in great detail. However, FIG. 1 does show the form of a suitable machine manufactured and sold by William Boulton Limited under the trade name Autoscroll. This machine comprises a container in the form of an annular bowl ll of generally semicircular cross section which is filled with ceramic particles in the form of prisms. The bowl is mounted on a plurality of springs (not shown) and is caused to vibrate in such a manner that the ceramic prisms, which surround the cams, progress around the bowl from the lower portion 12 thereof in a substantially toroidal helical path towards the outlet 13 from the bowl. The motion of the ceramic prisms is not a smooth progression, but a series of short jumps so that the prisms are constantly colliding with the cam shafts. A small amount of soapy water, sufficient only to wet all the ceramic prisms and the cam shafts, is added to the bowl to aid removal of particles of metal abraded from the cam shafts by the ceramic prisms.

The grinding process, as previously explained, leaves the surface of the cam with numerous sharp edges asperities between which are defined channels in which lubricant will be stored in use. These asperities are clearly seen in FIG. 2. The finishing operation does not remove the asperities, but rounds their sharp edges as can be seen in FIG. 3. Thus, the finishing operation produces a cam the profile of which is substantially unchanged by the finishing operation, and in which the total volume of the channels is substantially unaltered. Nevertheless, all the sharp edges have been rounded, so that there will be considerably less wear on the cam follower when the cam is in use.

The surface finish which is produced by the original grinding operation can of course be controlled, and the exact finish will depend on the particular application and will be obvious to an expert. Similarly, the length of the finishing operation will depend on the nature of the cam required for a particular operation, but can readily be deduced by an expert given the knowledge imparted by the present invention, which is that is is necessary to round the sharp edges without substantially reducing the volume of the channels. For any given ground cam which has a surface finish giving a satisfactory total volume of lubricant in use, then the life of the associated cam follower can be improved by subjecting the cam to the finishing operation described. Even if the cam is ground to a finish which would be regarded as totally unsatisfactory, the finishing operation will improve the cam so much that it may be satisfactory. For example, a cam for use with a conventional cam follower, manufactured from the material sold under the trademark Tufnol is usually thought to be reasonably satisfactory with a surface finish between 5 microinches and 32 microinches, but a cam ground to only 8 microinches is satisfactory if subjected to the finishing operation described using the apparatus described above for a period of about 2 hours.

Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

l. A method of manufacturing a cam for use in an ignition distributor, comprising grinding the cam to the desired profile, the grinding operation leaving the cam surface with sharp edges asperities defining therebetween channels which in use store lubricant, placing the cam in a container with abrasive particles and moving the container so that the particles collide with the cam surface and round off the sharp edges of the asperities without removing the asperities. 

1. A method of manufacturing a cam for use in an ignition distributor, comprising grinding the cam to the desired profile, the grinding operation leaving the cam surface with sharp edges asperities defining therebetween channels which in use store lubricant, placing the cam in a container with abrasive particles and moving the container so that the particles collide with the cam surface and round off the sharp edges of the asperities without removing the asperities. 